Update ClinicalTrials.gov to work with new API

[adam3smith]

The relevant documentation is here: https://clinicaltrials.gov/data-api/about-api/api-migration?expr=NCT03383315&fmt=JSON#query-full-studies
It doesn't look like the new API requires authentication, as previously suggested, so should be fine to migrate. There's a legacy endpoint, but that only supports XML and the translator is written for JSON, so we might as well figure out the new API (v2) format and save work on the data structure.